BIOLOGY Volume 3 - A Guide to General Biology - 2004

27. MECHANISMS OF SPECIATION

27.5. Natural Selection

The hypothesis of natural Selection, postulated by Darwin and Wallace, was based on historical data. Darwin believed that the time span required for the evolutionary change of a population must be too long for such change to be observed directly. Recent changes associated with the industrial, technical, and medical revolutions create such intense directional and disruptive selection pressures that we can now observe dramatic Changes in the genotype and phenotype of populations occurring quite rapidly. The discovery of Antibiotics in the 1940s created strong selection pressure in favour of bacterial strains possessing genetic resistance to antibiotics. Bacteria reproduce very rapidly, producing many generations and millions of individuals daily. As a result of a random mutation, a resistant Cell may appear, the descendants of which will thrive thanks to the absence of competition from other bacteria destroyed by the given antibiotic. In response to this, new antibiotics have to be created to destroy the resistant bacteria, and the cycle continues. Selective pressure is also created by The Use of substances such as DDT to control body lice and mosquitoes, and the anticoagulant warfarin to eradicate rats. Once resistance arises, it spreads rapidly throughout the population.

A classic example of evolutionary change is the response of certain moths to directional selective pressure caused by atmospheric pollution resulting from the Industrial Revolution. Over the past 100 years, more than 80 species of moths have developed dark forms, which are now found with varying frequencies throughout Great Britain. This phenomenon is known as industrial melanism. Prior to 1848, all described forms of the peppered moth (Biston betularia) were pale cream in colour with black dots and occasional dark speckles (Fig. 27.9). In 1848, a black form of this moth was discovered in Manchester, and by 1895, the peppered moth population in Manchester consisted of 98% black moths. This black 'melanic' form appeared as a result of repeated random Mutations, with the phenotype of the mutant individuals possessing a major selective advantage in industrial areas for reasons proposed and tested by Dr Kettlewell.

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Fig. 27.9. Polymorphism in the peppered moth (Biston betularia). A. Normal form Biston betularia typica. B. Melanic form Biston betularia carbonaria. (From E. B. Ford (1973) Evolution studied by observation and experiment, Oxford Biology Readers, 55, Oxford University Press.)

Peppered moths are active at night, and during the day they rest on tree trunks. The normal form has highly effective protective coloration that blends in with the Lichens covering the trunks. With The Development of the Industrial Revolution, sulphur dioxide produced by the combustion of coal caused the lichens to die off in industrial areas, exposing the dark tree bark, which became even darker due to the soot covering it (Fig. 27.10).

Fig. 27.10. Melanic and non-melanic forms of Biston betularia on tree trunks near Birmingham (A) and in Dorset (B). (By kind permission of Dr H. B. Kettlewell, Department of Zoology, University of Oxford.)

In the 1950s, Kettlewell marked a specific number of light and dark moths and released them in two locations: a polluted area near Birmingham, where the population consisted of 90% of the dark form, and an unpolluted area in Dorset, where the dark form was rare. Using a light trap, he recaptured the marked moths and obtained the following results:

Using cinematography, Kettlewell demonstrated that robins and thrushes feed on these moths, exerting a form of natural selection called selective predation, which in this case exerts selective pressure on the distribution of the melanic and light forms.

The results obtained indicate that the melanic form Biston betularia carbonaria has a selective advantage over the light form Biston betularia typica in industrial areas, whereas the light form has the advantage in unpolluted areas.

It was later established that the coloration of the dark form is determined by a dominant melanic allele. Fig. 27.11 shows the distribution of these two forms in the British Isles in 1958.

The presence of dark moths in the non-industrial areas of eastern England is explained by the dispersal of melanic forms and the prevailing westerly winds there. Following the enactment of the Clean Air Act in 1956, the proportion of light forms increased significantly once again as their elimination by selection in industrial areas decreased.

Fig. 27.11. Distribution of melanic and non-melanic forms of Biston betularia in the British Isles in 1958. (H. B. D. Kettlewell, Heredity, 1978, 12, 51-72).

27.5.1. Polymorphism

Polymorphism plays a significant role in The process of natural selection. It can be defined as the existence within a single population of two or more forms of a given species that differ in biochemical, morphological, or behavioural traits. There are two forms of polymorphism: transient polymorphism and balanced (or stable) polymorphism.

Balanced polymorphism

Balanced polymorphism is established when different forms coexist within a single population under stable environmental conditions. The most striking example of this is the presence of two sexes in animals and plants. The genotype frequencies of the various forms are balanced because both forms possess equivalent selective advantages. An example of balanced polymorphism in humans is the A, B, AB, and O Blood Groups. The frequencies of different genotypes may vary across different populations, but they remain constant from generation to generation within any given population. This is explained by the fact that no single genotype holds a selective advantage over the others. As statistical data show, white males with blood group O have a higher life expectancy than males with other blood groups, yet individuals with group O are more prone to developing duodenal ulcers than others, which, in the event of perforation, can lead to death. Other Examples of polymorphism include normal Vision and colour blindness in humans, the castes of workers, drones, and queens in social insects, and the pin-eyed and thrum-eyed forms in primroses.

A classic quantitative study of balanced polymorphism was conducted by Cain, Currey, and Shepherd on the common land snail Cepaea nemoralis. The shells of these snails can be yellow (appearing green if a live snail is inside), in various shades of brown including buff, pink, orange, and red. The Mouth of the shell may be dark brown, pink, or white, and the shell itself may feature up to five dark bands running along its contours (Fig. 27.12). Both shell colour and banding patterns are genetically determined. Coloration is determined by multiple alleles, with brown being dominant to pink, and both being dominant to yellow. Banding is a recessive trait.

Fig. 27.12. Variation in shell colour and banding patterns in Cepaea nemoralis. A — top view; B — side view. Showing the gradual transition from a yellow unbanded shell (top row, left) to a dark brown banded shell (bottom row, right). (After Tribe, Tallan, Erant (1978), Basic Biology Course, Book 12, Cambridge University Press.)

Thrushes prey on snails by carrying them to a nearby stone, which they use as an anvil to smash the shell and reach the snail. By studying the proportions of different shell types scattered around these anvils compared to snail habitats as a whole, Cain, Currey, and Sheppard demonstrated that selective pressures operate within the population. In areas with a relatively uniform Background, such as grass or leaf litter, unbanded yellow and brown shells possessed a selective advantage—fewer of these shells were found around the anvils (Fig. 27.13). Darker banded shells had the advantage where the background was variegated, such as in rough pastures and hedgerows. In any given habitat, thrushes prey most heavily on the most conspicuous forms. A large polymorphic snail population may occupy several microhabitats that differ in background appearance. Furthermore, background coloration and patterns can change with the seasons. Although predation of conspicuous forms occurs continuously, no single morph possesses an absolute selective advantage; consequently, the proportion of each form in the population remains relatively constant from year to year.

Fig. 27.13. Unbanded shells of Cepaea nemoralis against a background of leaf litter. The shell on the right is yellow, the top one is pink, and the two on the left are brown. (After E. B. Ford, Evolution studied by observation and experiment. Oxford Biology Reader, 55, Oxford University Press. 1973.)

The balance of frequencies among different morphs is not necessarily determined solely by coloration and banding patterns. Evidence suggests that physiological factors also contribute to maintaining this polymorphism equilibrium. In certain regions with dry calcareous soil and a light background, morphs with the least conspicuous coloration and banding do not always predominate. It is believed that in Cepaea, polymorphism is governed by a special type of linkage: the genes for color and banding are linked to form a supergene that Functions and is inherited as a single genetic unit. The constituent genes determine traits that confer selective advantages, thereby ensuring their persistence in the population. The Diversity of allelic forms for these genes, preserved through The high frequency of heterozygotes, forms The basis of polymorphism. This is reinforced by the linkage of genes determining certain physiological functions, which is also thought to help maintain balanced polymorphism. The presence within a single population of several distinct forms whose frequencies are too high to be accounted for by recurrent mutation is known as genetic polymorphism, with Cepaea being a prime example.

Transient Polymorphism

Transient polymorphism occurs when different forms, or morphs, coexist in a population undergoing strong selection pressure. The frequency of each phenotypic form is determined by the intensity of this selection pressure, as seen in the melanic and light forms of the peppered moth. Transient polymorphism is typically observed during the gradual replacement of one form by another.